Structures constructed of various materials are critical in many applications such as large space structures, oil rig platforms, industrial piers, microwave towers, marine structures, bridges, buildings, aircraft, spacecraft, turbine hubs and rotors, chemical manufacturing systems, power poles, and biomedical structures as used for example in monitoring the strength of bones and joints. Damage in these structures may cause severe economic consequences and possibly loss of human life. Moreover, the design characteristics of a passive or an active controller depend upon the mechanical properties of the structure to be controlled. If a change in mechanical properties occurs at any point in the structure, a corresponding adjustment should be made in the control system if the original control objectives are to be continually satisfied. Therefore, it is desirable to non-destructively monitor changes in the mechanical properties of structures. The locations and magnitudes of changes in mechanical properties may provide the basis for decisions regarding repairs or future uses of a structure.
To allow continued use of a tested structure, non-destructive damage detection (NDD) approaches are utilized. Many techniques of NDD are currently available, including but not limited to X-radiography, ultrasound, neutron radiography, eddy currents, optical holography, acoustic holography, and thermography. These techniques have been incorporated in various prior art approaches to NDD applied to large scale structural systems. However, the prior art approaches have several shortcomings. First, the prior art approaches are local (spanning at most only a few feet at a time) rather than global (spanning the entire structure) in their scope of damage detection. Second, the prior art approaches require the application of additional theoretical analysis (i.e., fracture mechanics or continuum damage theory) to define and assess identified damage. Third, they can only be applied to accessible portions of a structure. Fourth, the prior art approaches are extremely costly when applied to large scale structural systems.
Structural damage may be defined as deviations of geometric or material properties of a structure that may cause unwanted displacements, distortions or vibrations in the structure. One such measure of damage is stiffness loss at one or more locations in a structure. Stiffness loss may be detected by non-destructive vibration measurement techniques. Vibration techniques may locate flaws and defects that might otherwise allude other methods such as ultrasonics or visual inspection. Furthermore, vibrational techniques may increase the efficiency of overall inspection by first localizing areas of distress for more detailed examination by local methods.
For any structure, the natural modes of vibration depend only upon the mechanical characteristics of the structure and not upon the excitation. In many instances, the required modal vibration responses can be obtained by measuring the modal responses at only a single point on the structure. Using this method, operators may be alerted to handle life-threatening structural conditions or to confirm possible structural damage.
It is established that changes in the modal vibration response of a structure may reflect deterioration in that structure. However, there is considerable debate among practitioners regarding how to interpret the changes in the modal response to yield information on the location and magnitude of damage. Practitioners of vibration measurement have long been aware that decreases in the vibrational response frequencies of a structure indicate damage to the structure. However, prior art approaches incorporating vibration measurement techniques fail to accurately relate changes in vibrational response frequencies to the specific locations and severity of damage to the structure. Instead, many prior art approaches only detect the existence of damage in the structure. Recent approaches incorporating vibration measurement techniques have successfully related changes in vibrational response frequencies of a structure to the specific locations and severity of damage to the structure. However, even these approaches may be inaccurate or impractical in many instances, as for example when damage is extensive or when complex structures are analyzed.
Therefore, a need has arisen for an accurate and practical method and apparatus for damage detection.